Hydraulic grip



Jan. 6, 1970 F. F. EICHENBRENNER ETAL 3,437, 30

HYDRAULIC GRIP 2 Sheets-Sheet 1 Filed April 26, 1968 FIG. 2

POWER CYLINDER FIG. 6

POWER CYLINDER FIG. 5

INVENTORS FRED F. EICHENBRENNER WALER ILLG POWER CYLINDER TTORNEYS Jan.6, 1970 F. F. EICHENBRENNER E'I'AL 3,487,680

HYDRAULIC GRIP Filed April 26, 1968 2 Sheets-Sheet 2 POWER POWERCYLINDER CYLINDER FIG. 4 FIG 3 INVENTORS FRED F. EICHENBRENNER WALTERILLG TORNEYS United States Patent 3,487,680 HYDRAULIC GRIP Fred F.Eichenbrenner, Hampton, and Walter Illg, Yorktown, Va., assignors to theUnited States of America as represented by the Administrator of theNational Aeronautics and Space Administration Filed Apr. 26,1968, Ser.No. 724,551

Int. Cl. G01n 3/36 U.S. 01. 73-103 s 14 Claims ABSTRACT OF THEDISCLOSURE The invention described herein was made by employees of theUnited States Government and may be manufactured and used by or for theGovernment for governmental purposes without the payment of anyroyalties thereon or therefor. v

This invention relates to clamping arrangements for clamping sheet stockspecimens into mechanical testing machines and also to specifichydraulic clamps especially suited for this purpose. I

Prior art devices for fastening the ends of sheet metal specimens intothe grips of a testing machine have usually consisted of a pair of jawshaving a plurality of screws passing through them and arranged to drawthe jaws together and grip the specimens as they are tightened down.These prior art devices, while not wholly unsatisfactory, suffered fromseveral disadvantages, among these being the rotary strain induced inthe specimen as the screws are tightened, which in turn leads toupsetting of the stress patterns desired in axial loading fatigue tests.A second disadvantage was the nonrepeatability of the gripping pressuresince galling on the screw ends, deterioration of the threads, andvariation in lubrication would vary the effect of given torques applied,to. the individual screws. In addition, these devices were rather slowand laborious to use. l

Therefore, it is an object of the present invention to provide a meansof clamping sheetmetalspecimens into mechanical test machineswithoutinducingrotary strains in the specimen. c l

' It is a further object to provide a device capable of clamping sheetspecimens witha highly repeatable clamping pressure. 7

It is yet another rapidly and easily clamp. or release suchaspe cimeninto atestmachine. Y

Another object of the presentlinvention is to provide novel hydraulicclamps which are particularly. suited to the purpose of clamping suchspecimens into mechanical testing machines. T l I These and otherobjects which will become more apparent upon an inspection and'readingof the drawings and specification are accomplished by the usefofhydraulic clamping arrangements wherein a uniform clamping pressure isapplied with a minimum distortion of parts to the specimen to befastened. A

In the drawings, FIG. 1 is a side elevation in partial section of a testmachine with a sheet, specimen clamped therein.

object to provide a device; which will I,

FIG. 2 is a partial front elevation of the same assembly;

FIG. 3 is a partial view of a side elevation in partial section of atest machine with a specimen clamped therein with another form ofhydraulic clamp;

FIG. 4 is a front elevation of the same assembly;

FIG. 5 is a partial view of a side elevation of another form ofhydraulic clamp used in a test machine;

FIG. 6 is a partial view of a side elevation of another form ofhydraulic clamp used in a test machine.

Referring now to the drawings, FIG. 1 shows the basic layout of the testsetup for mechanical test of a sheet metal specimen. The specimen 10 isclamped at both its upper and lower ends with a hydraulic clamp assembly12, including a piston 14 disposed in a cylinder block 16. The piston 14has an elongated rod 18 secured thereto and passing through the cylinderend wall through the bore 20. Fluid seals 22 are provided in order toprovide a fluid-tight hydraulic chamber 24. Passageway 26 through thepiston is provided and is in communication with fluid line 28 and asource of fluid pressure 30, so that the chamber 24 maybe pressurizedwith a fluid pressure. Cylinder block 16 is formed with a flat end wall17, which acts as a reaction surface for the clamping action as to bemore fully described herein.

Clamping plate 32 is slidably disposed on a portion of the rod 18, andis retained thereon by the nut 34 which threadedly engages the end ofthe rod 18. The nut 34 provides a degree of adjustment to the travel ofthe clamping plate 32 to allow for specimens of varying thicknesses. Therod end may be formed with a hex head 40 thereon in order to looseneither the nut 34, or the locking ring 38 in case some binding in thethreads is encountered.

The end of the piston 14 is threaded at 36 in order to receive a lockingring 38 threaded on its interior to engage the piston 14.

The upper portion of the cylinder isformed with a flat plate 42 which isbolted to a load measuring device such as a weigh-bar 44,.which is inturn secured to the test machine frame 46. The lower hydraulic assembly12 has the plate .member 42 secured to the power cylinder assembly forinducing loads on the specimen 10. In use, the specimen to be tested isslipped over the projecting upper and lower rods 18, from which theclamping plates and force nuts have been removed. These elements arethen replaced and fluid pressure is introduced to line 28 pressurizingthe chamber 24 and causing the pistons 14 to move away. This produces aclamping action between the cylinder end walls 17 and the clampingplates 32 with a force magnitude directly related to the piston area andthe pressure level maintained in the chamber 24. It should be noted thatthe cylinder end wall 17 acting as reaction surface is held against.movement by theclamping plate 32 by the force of the fluid pressure inthe chamber 24 directed in the opposite direction.

After the desired clamping force is attained, locking rings 38 arehand-rotated until they engage the cylinder 16. Thefluid pressure isthen relieved to produce a solid mechanical lock at both ends of thespecimen, to allow the desired loadings to be induced.

' It should be pointed out that this lock has been accomplished withoutinducing rotary strain on the specimen since the hydraulic clamp 12operates with a straight pull, and the locking ring 38 is only rotatedby hand and no significant torque is applied thereto. Secondly, thisclamping pressure is highly repeatable since it varies only with. thepressure applied to chamber 24, and this parameter is rather easily andprecisely monitored and controlled. Thirdly, the clamping pressureapplied to the specimen is evenly distributed due to the low distortiondesign of the clamp. This is a result in part to the arrangement wherebyclamping forces are transmitted by a rod under tension rather than acompression, resulting in elimination of the usual substantial bucklingdeflections encountered in a piston and cylinder combination, and inpart to the countering of the reaction forces in the system to produce azero net force on the hydraulic assembly. Thus forces acting on cylinder16 to the left due to the clamping action of plate 32 are just balancedby the force to the right induced either by the fluid pressure on theend wall or the force ring 38. Hence, distortion and misalinement of theparts is minimized, resulting in an even pressure distribution acrossthe face of the clamping surfaces and elimination of racking loadsinduced by such an uneven pressure distribution. In addition, lateralloads on the weigh-bar 44 are similarly eliminated.

It is also seen that the process of installing a specimen is renderedmuch more rapid and is more easily accomplished than that utilizing themanually operated clamps.

A variation of this assembly is shown in FIGS. 3 and 4. The generalarrangement is the same, in which a specimen is fastened at its ends byhydraulic clamp assemblies 48. However, each hydraulic clamp is providedwith a plurality of piston and rod subassemblies slidably received inright and left cylinder block plates 50 and 52. Each subassemblyconsists of a piston 54 and a rod 56 secured thereto, the pistonreceived in a bore in the plate 50 to form a chamber 58. The rod 56passes through both plates through openings 60, and threadedly engages aforce nut 62, which may be received in a counterbore 64 formed in plate52.

Plates 52 and 50 are both secured to a plate 66 by means of cap screwand nut assemblies 68 passing through elongated holes 70, while thisplate is in turn secured to a load cell 72 which is secured to the testmachine frame.

Fluid pressure is supplied to each cell by means of a pressure source74, line 76, and interconnecting passages 78, 80, 82, 84 and 86 whichcommunicate with each chamber 58.

Each of the plates 50 and 52 are mounted so as to be slidable in trackassemblies 88, fastened to the upper plates 66. A pair of bars 90fastened to each of the plates 52 and 50 fitting into the trackassemblies may be engaged with bearing assemblies 92 so that each of theplates may be freely slid back from the position shown in FIG. 3 whencap screw and nut assemblies 68 are not in place.

In use, the plates 50 and 52 are Withdrawn to a point where the rod 56does not pass into plate 52, cap screw and nut assemblies and force nuts62 having been removed. The specimen 10 having been predrilled for thispurpose is then slipped over the rods 56 and plates 50 and 52 areadvanced into engagement. Cap screws and nut assemblies 68 and the forcenuts 62 are then installed to finger tightness. The introduction offluid pressure into each clamp assembly 48 causes a pressurizing of allof the chambers 58 to produce a desired clamping force on the specimen10.

This apparatus while incorporating the advantages ant. functioning ofthe single cell hydraulic clamp, possesses the additional advantage ofproducing a uniform clamping pressure simultaneously over a relativelygreat area, 7

This apparatus is similar to that shown in FIGS. 1 and 2 except thatinstead of a single clamping plate 32, a forked member 94 is used, andthe clamping action occurs between the tine members 96 and 98 instead ofbetween the clamping plate 32 and the cylinder. Thus, the fork '94 maybe secured to the power cylinder or load cell rather than the cylinderwhile keeping the specimen centered along the line of action of thepower cylinder, resulting in a somewhat smaller and simpler structure,while retaining the advantages of the basic hydraulic clampingarrangement. The time 98 acts as the reaction member and is maintainedagainst movement by the action of time 96 by the effect of thenullifying force applied by the cylinder block 16 produced either by theinternal fluid force or the locking ring 38.

A final variation is shown in FIG. 6. Here, a separate reaction plate102 is secured to the power cylinder, and the clamping plate is notconnected thereto, as in the forked arrangement of FIG. 5, allowingfreer clamping movement of the plate 100.

Therefore, it can be seen that an arrangement has been provided forclamping specimens into a test machine with a minimum of distortion ofthe specimens induced by the clamping process, with a highlycontrollable and repeatable clamping pressure, and which is applicableto specimens of considerable proportions. The process is also fast andrelatively effortless.

In addition, it can be seen that a novel hydraulic clamp has beenprovided in which distortion of the components is minimized, allowinggreat clamping pressures to be exerted without significant deflection ofthe parts which would lead to uneven pressure distribution across theclamping surfaces.

It should be understood the invention is not to be limited to theparticular embodiments disclosed, as these are intended to beillustrative of the invention. It also should be noted that some of theinventions advantages may be gained by the use of a single hydraulicclamp in a testing machine. The specimen may be first clamped at one endwith a prior art device, and since the other end is free, rotary strainsare largely avoided. Then the opposite end may be clamped with ahydraulic clamp according to the present invention, thus eliminating theinducement of rotary strain in the specimen.

Therefore, the invention is to be limited only by the following claims:

1. A hydraulic grip comprising:

block means having a passage formed therein;

a piston movably fitted into said passage and having a portion whichprotrudes out of said passage in a portion of its range of movement;

means for selectively producing a net fluid force on said piston anddisposed opposite said one direction;

, connector means drivingly connecting said piston and said clampingelement;

a reaction element interposed between said clamping element and saidpiston; and

means securing said reaction element against move ment by said clampingelement in response to said fluid force;

an enlarged element fitted over said piston; and

means for positioning said element against movement along said piston atselective points along said protruding portion of said piston.

2. The grip of claim 1 wherein said connector means includes a rodextending through said passage, and also includes engagement meansdrivingly connecting said rod to .said clamping element.

3. The grip of claim 1 wherein said positioning means comprises athreadedconnection between said piston and said enlarged element.

4-. The grip of claim 2 wherein said engagement means includes athreaded portion on said rod extending through said clamping element anda 9116? nut threadedly engaging said portion.

5. The grip of claim 1 wherein said reaction element comprises an endwall formed in said block means.

6. A hydraulic grip comprising:

block means having a plurality of spaced parallel passages formedtherein;

disposed therein drivingly connected to a first one of said members andmovable in response to fluid pressure therein, means for producing aclamping action between said elements in response to movement of saidslidable element, and supply means for first member, means for producinga clamping action between said elements in response to movement of saidslidable element, and supply means for introducing fluid pressure tosaid chamber; and

a plurality of pistons each slidably fitted into a respec- 5 introducingfluid pressure to said chamber means for tive passage; locking saidslidable element against movement in means for creating fluid chambersbetween one of the positions in its range of movement; and

respective piston ends of each piston and said block means for measuringloads induced on said specimen. means; 10 12. Apparatus for testing asheet specimen comprising: means providing fluid communication betweensaid fluid a pair of spaced elements;

chambers; power means selectively producing relative movement means forproviding a fluid pressure to said chambers; between said pair of spacedelements;

a clamping element spaced from said pistons; means for clamping portionsof said specimen to said means drivingly connecting each of said pistonsand 1 spaced elements, including at least one hydraulic said clampingelement; clamp comprising a pair of members, at least one a reactionelement in juxtaposition with said clamping of which is connected to oneof said spaced eleelement; and ments, and a hydraulic chamber having aslidable means for securing said reaction element against move elementdisposed therein drivingly connected to a ment by said clamping element,whereby a uniform first one of said members and movable in responseclamping force is created by each piston and passage to fluid pressuretherein, means for producing assembly by the introduction of fluidpressure to a clamping action between said elements in said chambers.response to movement of said slidable element, and

7. The grip of claim 6 wherein said clam-ping element supply means forintroducing fluid pressure to said is located with said chambers betweensaid pistons and chamber, and wherein said hydraulic clamp further saidclamping element, and said reaction element is interincludes at leastone other slidable element and posed between said pistons and saidclamping element. hydraulic chamber, and means drivingly connecting 8.The grip of claim 7 wherein said connecting means said at least oneother slidable element and said each includes a plurality of rods eachconnected to a refirst member, and wherein said supply means alsospective piston and passing through its respective fluid introducesfluid pressure to said at least one other chamber. fluid chamber.

9. The grip of claim 8, wherein each of aid C nn g 13. The apparatus ofclaim 12 further including means means further includes a portion of theIOd extending providing fluid communication between said fluid through apassage in said clamping element, a nut member h b threadedly engagingSaid Portion of the Tod and of a 14. Apparatus for testing a sheetspecimen comprising: larger size than said passage. a pair of spacedelements;

Apparatus for testing Sheet Specimen Comprising: power means selectivelyproducing relative movement a pair of spaced elements; between said pairof spaced elements;

power means selectively producing relative movement means f clampingportions of said specimen to said between said pair of spaced elements;spaced elements, including at least one hydraulic means for clampingportions of said specimen to said Clamp comprising a pair of ember atleast one Spaced elements, uding at least one hydraulic of which isconnected to one of said spaced elements, Clamp comprising a p ofmembers, at least n and a hydraulic chamber having a slidable element Ofwhich i Connected t One Of Said Spaced disposed therein drivinglyconnected to a first one ments, and a hy u i Chamber having a slidableof said members and movable in response to fluid element disposedtherein drivingly connected to a pressure h i means for producing aclamping first One Of Said members said Chamber interaction between saidelements in response to moveposed therebetween and movable in responseto fluid ment f id lid bl d d supply means pressure therein and theSecond of Said members for introducing fluid pressure to said chamber,and interposed between said slidable element and said means fmechanically locking said pair of bers against movement relative to eachother; and means for measuring loads induced on sald specimen.

References Cited means for measuring loads induced on said specimen. 11.Apparatus for testing a sheet specimen comprising:

a pair of spaced elements;

power means selectively producing relative movement between said pair ofspaced elements;

UNITED STATES PATENTS 2,481,684 9/1949 Rock 279-4 X 2,777,701 1/1957Hohwart et al. 279-4 X 3,323,357 6/1967 Gloor 73-103 spaced elements,including at least one hydraulic clamp comprising a pair of members, atleast one of which is connected to one of said spaced elements, and ahydraulic chamber having a slidable element JERRY W. MYRACLE, PrimaryExaminer US. Cl. X.R.

